I need to know the way to install a roof vent guard over a round roof
vent fan.

The round roof vent fan has a screen covering the air outlet. But the
screen has no frame to stiffen it. Consequently, squirrels push the
screen away easily. They lived inside the roof for some time until I
finially get rid of them. Now, I am sure that no squirrel is inside
my roof any more. I need to keep any new comer away.

I want to install a square steel cage over the round roof vent fan.
The problem is that I cannot find any instruction about installing it
anywhere in internet (they only say that it is easy to install).
Therefore, I am not sure if this is something that I can do. I don't
want to nail it in place because I am afraid that I may add a way for
water to leak inside the roof.

One thing that I want to point out is that I have a very low pitch
roof. The roof doesn't have any shingle. It is covered with a thick
layer of roll-roofing. I am not sure if this makes any difference or
not. I want to mention about this in case this makes any difference.

Please tell me the appropriate way to install a steel roof vent
guard. Thanks in advance.

Jay Chan